For many diagnostic purposes it is beneficial to remove bone shadows from a radiograph image. In the image with removed bone shadows, some findings can be made easier because bone shadows removed from the image no longer interfere with subtle soft tissue structures. For example, lung nodule detection can be improved by this technique by removing rib shadows. Removing bone shadows can be useful both for a human reader and a lung nodule CAD (Computer Assisted Detection) system, because the risk of missing lung nodules covered by rib shadows (false negatives) or identifying false nodules at rib-shadow crossings (false positives) is reduced.
A known method of obtaining better soft tissue images relies on using dual energy acquisition. However, using dual energy acquisition requires special x-ray equipment and a higher x-ray dose exposure to the patient.
Another approach is disclosed by Simkó et al in “Elimination of clavicle shadows to help automatic lung nodule detection on chest radiographs”, published in Vander Sloten et al. (Eds.) IFMBE Proceedings 22, 488-491, 2008. Here a clavicle contour is segmented and described by a parametric curve {x(s), y(s)}Sε[0, 1]. A neighborhood of the clavicle contour is defined as a set of the points (x(s), y(s)+d), wherein sε[0, 1] and dε{tilde over ([)}δ, +δ] for some positive number δ. A clavicle image is reconstructed by    1. Calculating the gradient image.    2. Building a bone model.    3. Recalculating the image from a modified gradient image
The bone model is built by smoothing in the {(s, d)|sε[0, 1] and dε{tilde over ([)}δ, +δ]} space, using 1-dimensional Gaussian kernels with a large standard distribution σs in the direction of the s variable, tangential with respect to the contour, and a small standard distribution σd in the direction of the y axis. The clavicle image is subtracted from the original image, yielding a soft tissue image with suppressed clavicle. A shortcoming of the above method is that it is assumed that the bone lies in a horizontal direction.
US 2009/0214099 discloses using a global affine transformation to reorient objects in an image. The bone shadows in the transformed image are then suppressed. For example, the method described by Simkó et al may be applied to suppress the clavicle oriented perpendicularly to the y axis. The image with suppressed clavicle is then transformed by the inverse of the global affine transformation to reorient objects in an image to their original positions. However, the method will not perform well when the contour of the bone is curved, which is substantially the case for every closed contour, for example.